In order to carry out gravure printing, it is generally necessary to provide so-called banks adapted to form independent fine recesses for holding ink on the surface of the gravure printing plate. It is well known that the banks adapted to form such fine recesses are formed by the use of the gravure screen. In general, the gravure screen has a number of independent opaque fine dots regularly aligned. If such a gravure screen is used to perform printing of more than two colors, the moire is liable to occur.
Also, there has recently been proposed a method for performing the gravure printing by use of a planographic or relief half-tone positive.
In a planographic or relief half-tone positive, the opaque dots are independent in the light portion to the middle tone portion but these dots are in contact with each other or superposed one above the other in the middle tone portion to the shadow portion.
Accordingly, in the case that the planographic or relief half-tone positive is used to make a gravure printing plate, the gravure screen is combined and used to prevent the flow of ink even in the middle tone portion to the shadow portion. Even in such a process as described, the moire sometimes occurs between the half-tone positive and the gravure screen.
It is known that the moire between the half-tone positive and the gravure screen is based on the interaction between the regular alignment of the dots of the half-tone positive and the regular alignment of the patterns of the gravure screen.
On the other hand, in the case of multi-color printing, a plurality of half-tone positives corresponding to various colors are necessary, and the aligning angles of the dots of these half-tone positives are different from each other. The reason is to avoid the formation of the moire between the half-tone positives, and thus the aligning angles of the dots of the half-tone positives are selected in such a manner that the moire is most imperceptible.
In general for foregoing reason, to render most imperceptible the moire pattern between the half-tone positive and the gravure screen in the case of multi-color printing, the gravure screen of the screen angle adapted itself to the aligning angle of the dots of the half-tone positive (the screen angle of the half-tone positive) is combined with the respective half-tone positive. Therefore, in the case of the multi-color printing, gravure screens in the number corresponding to the number of colors to be printed are prepared, or a gravure screen having a greater size than the exposure area actually required is used, the gravure screen being rotated so as to have a screen angle suitable for the respective half-tone positive. In the case of the former, there is a disadvantage in that a plurality of expensive gravure screens are necessary, and in the case of the latter, there is a disadvantage in that a gravure screen greater than the required exposure area is necessary.
Accordingly, the development of the gravure screen having no disadvantage as noted above has been expected.
Furthermore, where the design having regular patterns is present in an original even in the conventional gravure process, the moire sometimes occurs between the design and the gravure screen. Thus, the development of the gravure screen which forms no moire pattern as noted above has also been expected.